Viral Infections

Viral infections occur when viruses enter cells in the body and begin
reproducing, often causing illness. Viruses are tiny germs that can
reproduce only by invading a living cell.

KEYWORDS

for searching the Internet and other reference sources

Infection

Polymerase chain reaction

Virology

How Are Viruses Different from Bacteria?

Viruses are far smaller than bacteria. They are so small that they could
not be seen until the electron microscope was invented in the 1940s.
Unlike most bacteria, viruses are not complete cells that can function on
their own. They cannot convert carbohydrates to energy, the way that
bacteria and other living cells do. Viruses depend on other organisms for
energy. And viruses cannot reproduce unless they get inside a living cell.
Most viruses consist only of tiny particles of nucleic acid (the material
that makes up genes) surrounded by a coat of protein. Some have an outer
envelope as well.

Thousands of viruses

There are thousands of viruses, and in humans they cause a wide range of
diseases. For instance, rhinoviruses cause colds, influenza viruses cause
flu, adenoviruses cause various respiratory problems, and rotaviruses
cause gastroenteritis. Polioviruses can make their way to the spinal cord
and cause paralysis, while coxsackieviruses (sometimes written as
Coxsackie viruses) and echoviruses sometimes infect the heart or the
membranes surrounding the spinal cord or lungs. Herpesviruses cause cold
sores, chickenpox, and genital herpes, a sexually transmitted disease.
Other viruses cause a variety of conditions from measles and mumps to
AIDS.

The body's defense system

Most viruses do not cause serious diseases and are killed by the
body's immune system—its network of natural defenses. In
many cases, people never even know they have been infected. But unlike
bacteria, which can be killed by antibiotics, most viruses are not
affected by existing medicines. Fortunately, scientists have been able to
make vaccines, which help the body develop natural defenses to prevent
many viral infections.

How Do Viruses Infect the Body?

Viruses can enter the human body through any of its openings, but most
often they use the nose and mouth. Once inside, the virus attaches itself
to the outside of the kind of cell it attacks, called a host cell. For
example, a rhinovirus attacks cells in the nose, while an enterovirus
binds to cells in the stomach and intestines. Then the virus works its way
through the host cell's outer membrane.

After entering the cell, the virus begins making identical viruses from
the host cell's protein. These new viruses may make their way back
out through the host cell's membrane, sometimes destroying the
cell, and then attacking new host cells. This process continues until the
body develops enough antibodies
*
and other defenses to defeat the viral invaders.

Are Viruses Alive?

It would seem to be a simple
matter to tell
if something is alive. But biologists disagree on whether viruses are a
form of life.

Viruses lack certain features that other forms of life have. They cannot
convert carbohydrates, proteins, or fats into energy, a process called
metabolism. They cannot reproduce on their own, but must enter a living
cell and use the host cell's energy. On the other hand, like all
life forms, viruses do have genes made of nucleic acid that contain the
information they need to reproduce.

Biologists have an elaborate way of classifying every form of life. Each
is grouped into a kingdom (such as the Animal Kingdom) and smaller
sub-categories called the phylum, class, genus, and species.

Bacteria and fungi each have a kingdom of their own, but viruses are
left out of this system. Many biologists think that, unlike the forms of
life grouped into kingdoms, viruses did not evolve (develop) as a group.
Instead, viruses may have developed individually from the kind of cells
they now infect—animal cells, plant cells, or bacteria.

*
antibodies
are proteins made by the body's immune system to target a
specific kind of germ or other foreign substance.

Not all viruses attack only one part of the body, causing what is called a
localized infection. Some viruses spread through the blood-stream or the
nerves, attacking cells throughout the body. For instance, HIV, the human
immunodeficiency virus that causes AIDS, attacks certain cells of the
immune system that are located throughout the body.

How Long Do Viral Infections Last?

In most types of viral infection, the immune system clears the virus from
the body within days to a few weeks. But some viruses cause persistent or
latent
*
infections, which can last for years. In these cases, a person may get
infected and seem to recover or may not be aware of being infected at all.
Then years later, the illness will occur again, or symptoms will start for
the first time. Viruses that can cause latent infections include
herpesviruses, Hepatitis B and C viruses, and HIV.

How Do Viruses Cause Illness?

Viruses can cause illness by destroying or interfering with the
functioning of large numbers of important cells. Sometimes, as mentioned
earlier, the cell is destroyed when the newly created viruses leave it.
Sometimes the virus keeps the cell from producing the energy it needs to
live, or the virus upsets the cell's chemical balance in some other
way. Sometimes the virus seems to trigger a mysterious process called
"programmed cell death" or apoptosis (ap-op-TO-sis) that
kills the cell.

Some persistent or latent viral infections seem to transform cells into a
cancerous state that makes them grow out of control. It has been estimated
that 10 to 20 percent of cancers are caused by viral infections. The most
common are liver cancer caused by persistent infection with Hepatitis B or
Hepatitis C virus, and cancer of the cervix (the bottom of a
woman's uterus or womb), linked to certain strains of the human
papillomavirus.

*
latent
infections are dormant or hidden illnesses that do not show the signs
and symptoms of active diseases.

Sometimes a viral illness is caused not by the virus itself, but by the
body's reaction to it. The immune system may kill cells in order to
get rid of the virus that is inside them. This can cause serious illness
if the cells being

There are thousands of kinds of viruses. Most consist only of tiny
particles of genetic material surrounded by a coat of protein and
sometimes an outer envelope. Specific viruses attach themselves to the
outsides of specific host cells, and then work their way inside
through the host's outer membranes. Once inside their host
cells, the viruses reproduce. The new viruses can destroy their host
cells and then move on to attack new host cells.

killed are very important to the body's functioning, like those in
the lungs or central nervous system, or if the cells cannot reproduce
quickly enough to replace the ones being destroyed.

An individual adenovirus viewed under an electron microscope. Viruses
are so small that they could not be seen until the electron microscope
was invented in the 1940s. This one was photographed at 800,000 times
its original size.

How Are Viral Infections Diagnosed and Treated?

Symptoms

Symptoms vary widely, depending on the virus and the organs involved. Many
viruses, like many bacteria, cause fever, and either respiratory symptoms
(coughing and sneezing) or intestinal symptoms (nausea, vomiting,
diarrhea). Viral illnesses often cause high fevers in young children, even
when the illnesses are not dangerous.

Diagnosis

Some viral infections, such as influenza, the common cold, and chickenpox,
are easily recognized by their symptoms and no lab tests are needed. For
many others, such as viral hepatitis, AIDS, and mononucleosis, a blood
sample is analyzed for the presence of specific antibodies to the virus.
If present, these antibodies help confirm the diagnosis. In some cases, a
virus may be grown in the laboratory, using a technique called tissue
culture, or identified by its nucleic acid, using a technique called
polymerase chain reaction (PCR). Tests like PCR or tissue culture are used
when antibody tests are not precise enough or when the actual amount of a
virus in the body must be determined.

Treatment

Viruses cannot be treated with the antibiotics that kill bacteria.
Fortunately, a few drugs, such as ribavirin and acyclovir, can control the
spread of viral invaders without destroying host cells. Intense research
to find better treatments for AIDS has led to development of many drugs
that help fight the virus. Unfortunately, none of these drugs has been
able to treat viral infections as effectively as antibiotics treat
bacterial infections.

How Are Viral Infections Prevented?

Hygiene and sanitation

The first step in preventing the spread of viral infections is simply to
practice good hygiene. This means washing the hands often, and eating only
food that has been prepared properly. It also means building and
maintaining facilities for getting rid of sewage safely and for providing
clean drinking water.

Vaccination

Another important preventive measure is immunizing people against viruses.
This involves giving people vaccines that stimulate the immune system to
make antibodies, proteins that target a specific germ. Vaccines to prevent
Hepatitis B, polio, mumps, measles, rubella (German measles), and
chickenpox are usually given to babies and young children in the United
States. Vaccines also can prevent influenza and Hepatitis A.

What Is a 24-Hour
Virus?

When people have a mild illness—perhaps fever and an upset
stomach, perhaps nausea and diarrhea—they often say they have a
"24-hour virus" or a "stomach virus." Many
viruses can cause these kinds of symptoms, but there are many other
possible causes as well, including bacterial infection or bacterial food
poisoning. People usually recover from these brief or mild illnesses
before doctors can do the tests that determine the causes. So a
"stomach virus" may or may not be a virus at all.

Vaccines are useful only against certain kinds of viruses. For example,
the polioviruses that cause poliomyelitis (polio), a great crippler of
children in the past, are few in number and relatively stable. So it was
possible in the 1950s to make a vaccine that protects children from
getting
polio (although the illness still occurs in the developing world where
fewer children are vaccinated). On the other hand, influenza viruses
change in minor ways every few years and in a major way about every ten
years, so a flu vaccine is useful for only a year or two.

One reason a vaccine for the common cold has never been developed is that
there are at least a hundred different rhinoviruses that cause colds, and
so far it has not been possible to make a vaccine that works against all
of them. A similar problem with HIV, which has many different and
fast-changing strains (variations), is one of several reasons why progress
toward an AIDS vaccine has been slow.

User Contributions:

The text cuts off before the illustration: Sometimes a viral illness is caused not by the virus itself, but by the body's reaction to it. The immune system may kill cells in order to get rid of the virus that is inside them. This can cause serious illness if the cells being

There are thousands of kinds of viruses. Most consist only of tiny particles of genetic material surrounded by a coat of protein and sometimes an outer envelope. Specific viruses attach themselves to the outsides of specific host cells, and then work their way inside through the host's outer membranes. Once inside their host cells, the viruses reproduce. The new viruses can destroy their host cells and then move on to attack new host cells.

Since contracting swine flu in 2009,I have not felt well and I have had terrible respiratory problems! Dr's do not seem interested and I have become resigned to the fact I may always be ill. I have had the worst year yet since Jan 2012 I have been ill and admitted to hospital on several occasions. Where do I get the help I need?I am 51.
Denise

This was great information, HOWEVER, it would be helpful to suggest how to keep our antibodies up for the battle. Yes, obviously good food and exercise etc. I do not trust the government to provide a healthy vaccine and so I must risk it by keeping antibodies up. If only there was confidence in the government. And I hope someone answers 'denise's' question.

Comment about this article, ask questions, or add new information about this topic: